1. Field of the Invention
Devices for nuclear medicine imaging.
2. Description of the Related Art
Scintillation camera technology has serious limitations in nuclear imaging applications. These devices are extremely large, bulky, and fragile, and require careful and routine quality control monitoring in order to maintain acceptable image quality. These characteristics have limited many applications of nuclear imaging in medicine.
Pressurized xenon multiwire proportional detectors for medical imaging have been explored by several groups, including Proportional Technologies, Inc. (Bolon, IEEE Transactions on Nuclear Science, Vol. 36, No. 1, 661-664, 1978; Anisimov, Nucl. Instr. Meth. A235:582-588, 1985; Bellazzini, Nucl. Instr. Meth. 228:193-200, 1984; Lacy, J. Nucl. Med. 256:1003-1012, 1984; Lacy, Nucl. Instr. Meth. A269:369-376, 1988; Lacy, J. Nucl. Med. 29:293-301, 1988; Verani, Amer. J. Card. Imag., Vol. 2, No. 3:206-213, 1988; Adams, J. Nucl. Med. 31:1723-1726, 1990; Verani, J. Am. Coll. Cardiol. 29:1490-1497, 1992; Verani, J. Am. Coll. Cardiol. 19:297-306, 1992). In order to achieve acceptable efficiencies at relevant gamma energies, such devices as have been proposed must be thick and operate at high pressures. Ionization must therefore be collected over long drift distances to the active anode avalanche grid without losses and without substantial dispersion of the electron cloud. Electron dispersion can cause serious fluctuation and lengthening of the delivered avalanche signal while losses through attachment to electronegative gas contaminants can seriously impair energy resolution and charge proportionality. Also, pressure must be adequate so that the energetic photoelectron range is small in the gas relative to the spatial resolution desired. Such operation has proved to be feasible at moderate pressures up to 3 atmospheres (atm) and modest gamma energies (Lacy, J. Nucl. Med. 256:1003-1012, 1984). Ionization is efficiently collected over a 5 cm active detector depth and avalanche signals with little or no dispersion are obtained at energies up to and slightly above 60 keV. However, the maximum practical detector pressure, imposed by the large entrance window of this camera design, limits application to imaging by this camera to a radiopharmaceutical the isotope of which is Ta-178(55, 64 keV). With a camera of this design a non-competitive efficiency of less than 30% is achieved for T1-201 while for Tc-99m, the efficiency is less than 5%. Although image quality is good for T1-201, for Tc-99m, the photoelectron range is on the order of 1.8 cm, preventing accurate localization and causing a very lengthy and variable avalanche signal as the extended electron cloud is collected. Even for Ta-178, the improvements for a camera of this design are desirable. Overall efficiency for this low energy isotope is only 50%. Both patient dose and radiopharmaceutical cost could be halved if camera efficiency could be increased to near 100%.
Attempts to push detector pressures to significantly higher levels, in order to achieve improved high energy efficiency and more compact ionization deposition, have failed. Bolon et al., supra, have reported one of the more significant attempts in this direction (1978). In Bolon's work, a detector was constructed and operated at 6 atm with a 90% xenon/10% methane gas mixture. Efficiency of this device for Tc-99m was only 20%, despite its sizable depth of more than 10 cm. Charge collection performance was also disappointing. Owing to the substantial collection distances, energy resolution was suboptimal for T1-201 and essentially absent for Tc-99m. From Bolon's report it clearly appears that much higher pressures than 6 atm are required for acceptable performance with Tc-99m, and large collection distances cannot be tolerated if good energy resolution is required at such pressures.
A high speed xenon multiwire detector, developed by Proportional Technologies, Inc. (PTI), and now described in U.S. Pat. Nos. 4,870,282 and 4,999,501, provides unique and advantageous characteristics for in vivo cardiac imaging. Within the last decade it has been commercially developed into a portable nuclear medicine camera system, which has been utilized extensively at the Baylor College of Medicine, The Methodist Hospital and The Philadelphia Heart Institute in the nuclear cardiology field (Lacy, supra, 1988; Verani, J. Am. Coll. Cardiol. 19:297-306, 1992; Gioia, Am. Heart J., 130:1062-1067, 1995). It is currently being evaluated at several clinical sites; including Yale School of Medicine, Hahnemann Hospital of Allegheny University, and Hartford Hospital; for use in assessment of global and regional ventricular function. In these applications, the PTI high speed xenon multiwire detector is used to image the generator-produced radiopharmaceutical, Ta-178, which is intravenously injected and utilized as a blood label. Via this approach, high-quality, first-pass imaging of the heart is now performed (Lacy, supra, 1988; Lacy, J. Nucl. Med. 32:2158-2161, 1991). In over six years of clinical use, this camera, the Multiwire Gamma Camera (MWGC), has proved to be a reliable and practical device, and especially valuable in post-intervention evaluation of ventricular performance (Verani, supra, 1992; Gioia, supra, 1995). It, the MWGC, is very rugged, requires practically no maintenance, and has a relatively low cost.
The MWGC device, as generally shown in FIG. 1, has a 25 cm diameter circular sensitive area, and a 5 cm working volume depth. It operates at three atmospheres absolute pressure (3 atm) with a xenon/methane (90/10) gas mixture. The MWGC consists of two drift regions and a detection region, which is contained within an aluminum pressure vessel having a thin aluminum entrance window of spherical shape. X-rays entering through the aluminum window interact with the pressurized gas. The resulting ionization is impelled to the detection region by a drift field of 2000 volts/cm. The drifted ionization is collected at the anode, where the charge is amplified by gas avalanche.
Position determination of the anode avalanche is obtained by detection of the signals induced in the two cathode grids, which are oriented orthogonally to each other (Lacy, NIM, 1974). Each wire of each cathode grid is attached to a tap of a discrete delay line, and position is sensed by measurement of the delay time between occurrence of the avalanche on the anode grid and arrival of the signal at the ends of the cathode delay lines. The use of very high-speed delay lines (delay 10 nsec/cm), combined with the intrinsically fast gas avalanche process provides high rate operation with a peak count rate of 850,000 cps. Spatial resolution is defined by the 2 mm wire spacing of the detector grids. Because of the digital nature of the readout system, images are highly uniform and distortion free. Intrinsic spatial resolution of the device is 2.5 mm FWHM.
The pressurized xenon multiwire proportional camera (MWGC) has found successful application in medical imaging of the short-lived isotope Ta-178. The MWGC camera developed by Proportional Technologies, Inc.(PTI) for this purpose has received 510(k) premarket authorization (#K963730), and Phase III FDA studies sponsored by PTI are underway for the Ta-178 radiopharmaceutical under IND #51666. The design of the MWGC, however, has limitations which prevent broader application of the MWGC device with other radiopharmaceuticals, thus narrowing its focus as previously discussed. For competitive imaging of T1-201, a pressure of 10-20 atm is required while for Tc-99m, a pressure approaching 60 atm is needed. The photoelectron range in Xenon for these isotopes is shown in FIG. 2 as a function of Xenon pressure in atmospheres (atm).